The present invention generally relates to rotary recording mediums, and more particularly to a rotary recording medium recorded with information signals on a continuous spiral track which is formed on the same side of the rotary recording medium, in which tracks which are recorded with digital signals mainly including audio information and tracks which are recorded with analog signals mainly including a composite video signal coexist on the rotary recording medium.
There is a known type of a rotary recording medium (hereinafter simply referred to as a disc) which is recorded with analog information signals such as a composite video signal and audio signals. In such a disc, the analog information signals are subjected to a frequency modulation, for example, and are recorded on a spiral track or concentric tracks on the disc as variations in geometrical configuration, for example. This disc is often referred to as a video disc because the recorded information mainly includes the composite video signal. The composite video signal or the like is recorded on the tracks on the video disc in the form of modulated signals which are obtained by modulating a carrier by the analog information signals. An address signal which is used for random access and the like, is recorded within a predetermined duration within a vertical blanking period of the composite video signal, for example. The address signal itself is an encoded digital signal, however, the main information recorded on the video disc is the analog modulated composite video signal or the like. Hence, in the present specification, tracks such as the recorded tracks of this video disc, will be referred to as "analog recorded tracks" for convenience' sake.
On the other hand, there is another type of a disc which is recorded with digital signals. In such a disc, audio signals or audio and video signals are subjected to a digital modulation, and are time-sequentially multiplexed and recorded on concentric tracks or a spiral track on the disc as variations in geometrical configuration, for example. This disc is often referred to as a digital audio disc because the recorded information mainly includes the audio signals, and the video signal mainly relates to a still picture and is simply recorded as a supplementary information to help the listener with his imagination. The audio signals or the audio and video signals are recorded on the tracks on the digital audio disc after being converted into the form of digital signals which are obtained by subjecting the audio signals or the audio and video signals to a digital modulation and then subjecting the digital modulated signals to a frequency modulation or the like. In the present specification, tracks such as the recorded tracks of this digital audio disc, will be referred to as "digital recorded tracks" for convenience' sake.
In an electrostatic capacitance type video disc proposed in a U.S. Pat. No. 4,331,976, the recorded signals are reproduced from the video disc by detecting the variations in the electrostatic capacitance which is formed between the video disc and an electrode of a reproducing stylus. Reference signals for tracking control, are recorded on both sides of the information signal recorded track (analog recorded track). According to this video disc, the need for a stylus guide groove was eliminated by use of the reference signals, is known. In this known video disc, a tracking control was carried out with respect to the reproducing stylus so that the reproducing stylus accurately scans over the information recorded track during the reproduction mode, by comparing the levels of the reference signals which are reproduced from the video disc.
On the other hand, an electrostatic capacitance type digital audio disc is also known. The electrostatic capacitance type digital audio disc has no stylus guide groove and is recorded with reference signals for tracking control on both sides of the information signal recorded track (digital recorded track), similarly as in the case of the electrostatic capacitance type video disc. During the reproducing mode, this electrostatic capacitance type digital audio disc is rotated at a predetermined rotational speed which is the same as the rotational speed of the electrostatic capacitance type video disc. The frequencies of the reference signals and the method of reproducing the reference signals, are the same between the electrostatic capacitance type digital audio disc and the electrostatic capacitance type video disc. Further, in both the electrostatic capacitance type digital audio disc and the electrostatic capacitance type video disc, the recorded signals are reproduced from the disc by detecting the variations in the electrostatic capacitance between the disc and the electrode of the reproducing stylus. For these reasons, even when the digital audio disc is played on a video disc reproducing apparatus which is designed to play the video disc, the tracking control is carried out with respect to the reproducing stylus similarly as in the case where the video disc is played, and the recorded signals can be picked up and reproduced from the digital audio disc by the reproducing stylus. The signals which are reproduced from the digital audio disc, are demodulated into original audio signals or the like in an adapter which is coupled to the video disc reproducing apparatus.
Accordingly, the previously proposed electrostatic capacitance type video disc and the electrostatic capacitance type digital audio disc can be played on the same electrostatic capacitance type video disc reproducing apparatus. In other words, the above video disc and the digital audio disc can be played compatibly on the same video disc reproducing apparatus.
However, the digital audio disc and the video disc were mutually independent discs, and the compatibility did not exist in the true sense of the word. On the other hand, the digital audio disc is recorded with digital signals. Thus, compared to the video disc, the audio signals are reproduced from the digital audio disc with a wide dynamic range and with an extremely high fidelity, due to the characteristics of the digital signal transmission. Moreover, the still picture which is reproduced from the digital audio disc is extremely sharp, and there is of course an advantage in that the audio signals are reproduced from the digital audio disc with an extremely high fidelity together with the still picture. On the other hand, the still picture is reproduced from the video disc by repeatedly reproducing the same track on the video disc. Generally, the audio signals are muted during the still picture reproduction, and it is impossible to simultaneously reproduce the audio signals and the video signal from the video disc. However, due to the analog signal transmission in the video disc, it is possible to transmit the information signals in real time with a frequency band in the range of several MHz according to the video disc. Thus, compared to the digital audio disc in which the information signals are transmitted with a frequency band in the range of several tens of kHz in order to improve the transmitting accuracy, the video disc is advantageous in that it is possible to simultaneously reproduce a moving picture and the audio signals. Accordingly, in order to ensure optimum reproduction of the recorded signals it is desirable to select and record one of the digital signal and the analog signals depending on the information contents.